Topics Covered

Background

Graphene is a single atomic sheet of carbon atoms in the arrangement found in
graphite. The fact that it is thermodynamically stable as a single layer allows
the exploitation of its unique electron transport properties. In particular, the
two-dimensional electronic properties of graphene presents the potential for it
to function as a conductor, a transistor, a quantum dot, a molecular switch, or
other devices. The lateral confinement of the layer, however, is key to causing
these behaviors to be expressed. It is also very important in researching these
phenomena to have the ability to control, based on top-down designs, where the
patterning is done in relationship to probes and external connections.

Current Techniques Available for Pattering Graphene

At the present time patterning of graphene is accomplished by techniques such
as STM or electron beam lithography, but these suffer from throughput and
process control constraints which limit the achievable repeatability of the
structures. Therefore it would be attractive to directly pattern the necessary
structures with good control. In addition, a complete solution also requires
high resolution imaging in order to place the cuts and verify the results.

Imaging and Machining Graphene Using The ORION® PLUS from Carl
Zeiss

The ORION® PLUS makes it possible both to image and machine
graphene in one seamless operation. The high resolution of the microscope – a
world record value for surface imaging at 0.25 nm – combined with its extreme
surface sensitivity allows the imaging of small graphene features with single
monolayer detection. There is in addition a benefit with the helium ion beam in
that it can gently machine graphene. A low sputter yield, estimated to be
between 0.006 and 0.02 carbon atoms per helium ion, depending on sample
thickness and substrate type, means that imaging can be done safely in a lower
dose regime while nano-scale milling can be done at higher dose. By toggling the
ion optics configuration between these two modes one can carry out the entire
device formation process: identifying the area of interest, defining the
machining geometries, making the cuts and inspecting the result.

Figure 1 demonstrates both the high surface sensitivity for imaging graphene
and the speed control for material removal. The image reveals a lot of surface
detail on the sample. A 500 nm area in the center of this image was subjected to
a higher dose of helium ions at 38keV. This facilitated the removal of a small
amount of material from the surface. Inspection shows how the topography was
made flat but without significant milling into the surface.

Figure 1. ORION® PLUS image of graphene layers (thickness
not determined) showing surface modification. A small ion dose was applied to
the outlined area. A thin surface layer could selectively be removed.

Figure 2 shows a complete breakthrough milling operation. In this case a much
larger dose was applied to a 100nm square area, making a cleanly machined
feature all the way through the layers. This type of machining is not possible
with an SEM, because there is no sputtering. It is also beyond the
reach of traditional gallium FIB, which would create too much damage and would
also introduce metallic contaminants into the feature. Since the sputter yield
is so low, it is possible to obtain a high signal to noise image at an ion dose
two orders of magnitude below what is required to remove a single monolayer of
graphene – meaning that imaging is non-destructive to the sample. We have
demonstrated by this process the ability to create a 20nm wide ribbon in a
single layer of graphene – a requirement for testing two-dimensional
semiconducting properties in this material.

Figure 2. ORION® PLUS image of graphene layers showing
ion milling. The small square area in the center is a 100nm box machined
completely through the material. The edge acuity of the box demonstrates the
excellent achievable resolution which can be achieved for patterning.

Application

High spatial resolution patterning of graphene layers for research and
prototyping in semiconductor or material science fields. This is for the
creation of devices of nanometer dimensions that will exhibit new useful
properties due to electron confinement.

ORION® PLUS Capabilities

High resolution and surface sensitivity to image graphene sheets; the ability
to select either imaging or machining modes; non-contaminating operation.

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